The technical area whereto the present invention belongs is that of monitoring of drilling wells for the exploitation of the hydrocarbon resources of the subsoil.
Among the activities of monitoring of the wells special importance is represented by the analysis of the gases extracted from the drilling muds. This analysis supplies information both on the nature of the state of the drilled rock and on the possible hydrocarbon resources present.
All the gases extracted from the drilling muds comprise both hydrocarbons and non-hydrocarbon species. Among the latter particular importance is assumed by hydrogen sulfide H2S, due to its toxicity to humans and its aggressiveness and corrosiveness for equipment. For this reason, when the presence of H2S is detected or presumed in the subsoil during drilling, additives are fed into the drilling mud, known in the art as “scavengers”, which, reacting with the H2S in gaseous form, form solid precipitates, in this way neutralising the toxic and aggressive action of the H2S.
In addition to the reasons stated above, relating to the hazardous nature of the H2S, detection of the presence of the H2S is important for better characterisation of the zone of exploration from the geological viewpoint and obtaining, in this way, information useful for exploitation of the hydrocarbon resources.
At the state of the art, one of the best known systems for the detection of the H2S in the drilling mud is the device known in the art as “mud duck”, described in Calmer S. H. “H2S Detector Aids Drilling Safety”, Oil and Gas Journal, page 160, Nov. 19, 1979 and in Hadden D. M. “A system for continuous on-site measurement of sulphides in water-base drilling muds”, Proc. SPE Sour Gas and Crude symposium 1977. This system comprises a probe which contains three electrochemical sensors: two for the measurement of the concentration of the ion species derived from the reaction of the H2S with the drilling mud and a third sensor for measuring temperature. Since said mud is generally characterised by a basic pH, the gaseous H2S present in the subsoil reacts with it, giving rise to the formation of the HS− and S2− ion species, which remain dissolved in the mud itself. The first of the sensors of the mud duck system therefore measures the concentration of the hydrogen ion [H=], and the second the concentration of the sulphide [S2−] ions. The concentration of the hydrogen sulphide [HS−] ion is calculated from the concentration of the sulphide ions [S2−] and of the hydrogen ion [H+], by means of the following well known equation:
                              K          1                =                                                            [                                  H                  +                                ]                            ⁡                              [                                  S                                      2                    -                                                  ]                                                    [                              HS                -                            ]                                ≈                      10                          -              13                                                          (        i        )            where [H+], The concentrations of hydrogen ion [H+], and of the sulphide [S2−] ions are, in their turn, not the values directly measured by the sensors, but are estimated starting from the values directly measured by the sensors through the following well known Nernst equations:
                              E                      S                          2              -                                      =                              E            0                    +                                    RT                              n                ⁢                                                                  ⁢                F                                      ⁢                                          log                e                            ⁡                              [                                  S                                      2                    -                                                  ]                                                                        (        ii        )                                          E                      H            +                          =                              E            0                    +                                    RT                              n                ⁢                                                                  ⁢                F                                      ⁢                                          log                e                            ⁡                              [                                  H                  +                                ]                                                                        (        iii        )            
where E0 is the reference potential, n the charge of the ion being measured, F the Faraday's number, R the gas constant and T the temperature in ° C.
One of the disadvantages of the mud duck system arises from the fact that, in order to measure the concentration of the H+, and hence of the HS−, and S2− species, the probe of the mud duck is immersed in the drilling mud collected in the tank of the vibrating screens. For greater clarity it is recalled here that when the drilling mud rises to the surface it flows along a pipe known as “flow line” which ends in a tank where a system of vibrating screens separates the fluid component of the mud from the drilling debris, said fluid component being then destined to be returned into circulation in the drilled hole. This tank constitutes an environment strongly unfavourable to measurements performed by the probe of the mud duck system. It is in fact not unusual for this probe to be contaminated to the extent of becoming unusable, unless adequately cleaned. For a correct use of the mud duck it is therefore necessary to provide for regular maintenance for the cleaning of the probe.
A second system, known in the art as “Garrett Gas Train”, and described in Garrett L., Clark R. K., Carney L. L. and Grantham C. K. “Chemical Scavengers for Sulfides in Water Based Drilling Fluids”, Journal of Petroleum Technology, June, 1979, page 787, provides for the filtrate of the drilling mud to be subjected to acidification, so that the hydrogen sulphide (HS−) and sulphide (S2−) ions react, giving rise to free H2S. The latter is transported by a carrier gas into a special chamber where, by means of colorimeter vials, an indication is obtained of the quantity of H2S present in the mud. It is clear that such a type of measurement, while having the advantage, with respect to the previous one relating to the mud duck system, of having been performed in an isolated environment, does not have high precision, due to the inevitable intrinsic limits of the colorimeter vials. The principle of functioning of the latter is based, in fact, on the change in colour of an appropriate reagent, following the reaction with the H2S. The length of the coloured part of the vial is proportional to the concentration of the H2S. The estimate of the latter is therefore made by comparing visually the length of the coloured part with a concentration scale printed on the vial. A quantification of this type is evidently subject to the errors which always accompany every measurement which is based on valuations performed by a human operator.